DE102020004878A1 - Process for producing supported metal nanoparticles - Google Patents

Abstract

The present invention relates to a method for producing supported metal nanoparticles, in which nanoparticles of a metal dispersed in a solvent are deposited onto a support material by means of a salt.

Description

The present invention relates to a method for producing supported metal nanoparticles, in which nanoparticles of a noble metal, dispersed in a solvent, are deposited onto a support material by means of a salt.

Catalysts containing metals, in particular catalysts containing noble metals, are used on a large scale to clean exhaust gases from internal combustion engines. Thus, both three-way catalytic converters for cleaning exhaust gases from gasoline engines and diesel oxidation catalytic converters contain noble metals, in particular platinum group metals such as platinum, palladium and rhodium, deposited on high-surface carrier oxides. But other precious metals such as silver and gold are also used.
Such catalysts are prepared according to conventional prior art methods, in that an aqueous solution of a noble metal salt is impregnated onto a support oxide, for example aluminum oxide. However, catalysts produced in this way are disadvantageous in that they age over time and in particular as a result of the influence of high temperatures, ie they lose catalytic activity. This effect can be attributed in particular to agglomeration of the noble metal particles and thus to a loss of catalyst surface area.
This disadvantageous effect is significantly lower if the precious metals are present on the carrier material in the form of nanoparticles that are as homogeneous as possible. The associated increased catalytic effect and improved aging stability of the catalysts allow, in particular, smaller amounts of the expensive precious metals to be used.

the W02017/118932 A1 discloses a diesel oxidation catalyst comprising platinum group metal in the form of nanoparticles and a metal oxide as a support material. To produce the diesel oxidation catalyst, the nanoparticles are introduced into a washcoat in the form of a nanoparticle dispersion, with which a carrier substrate is coated. The carrier material can be contained in the same washcoat or the carrier material is applied separately to the carrier substrate in a first coating step, followed by coating with the washcoat containing the nanoparticle dispersion. According to WO2017/118932 A1 the nanoparticle dispersion is produced chemically, ie starting from a precursor compound of the platinum group metal with the aid of reducing and stabilizing agents.

However, nanoparticle dispersions of noble metals can be prepared more simply and economically by pulsed laser ablation in a solvent. In this process, a pulsed laser beam is focused onto the surface of a solid precious metal that is immersed in a solvent. In the process, material is removed from the solid noble metal, which forms nanoparticles or a nanoparticle dispersion in the solvent, see for example W02010/087869 A1 .

the WO 2012/080458 A1 discloses a method for producing micronanocombined active systems in which nanoparticles of a first component are connected to microparticles of a second component. According to this method, a colloid suspension containing nanoparticles of the first component, which is preferably produced by means of laser ablation, is intensively mixed with microparticles of the second component, so that the nanoparticles adsorb onto the microparticles.

The production of a catalyst by supporting the metal nanoparticles from a dispersion produced by laser ablation on a support oxide is difficult insofar as re-agglomeration of the nanoparticles inevitably occurs during the process steps. The control of the nanoparticle size in the catalyst is difficult if not impossible.

The object of the present invention is therefore to provide a method for producing metal nanoparticles adsorbed on a carrier oxide, in which re-agglomeration of the nanoparticles can be largely prevented.

It has now surprisingly and unforeseeably been found that this object is achieved by adding the carrier oxide to a dispersion containing metal nanoparticles obtained by laser ablation in an organic solvent and then adsorbing the nanoparticles on the carrier oxide by adding a salt which is insoluble in the solvent.

• • Bereitstellen einer durch Laserablation erhaltenen Dispersion von Metallnanopartikeln in einem organischen Lösungsmittel;
• • Zugabe des Trägeroxids zu der Dispersion; und
• • Adsorption der Metallnanopartikel auf dem Trägeroxid durch Zugabe eines im organischen Lösungsmittel unlöslichen Salzes.

The present invention accordingly relates to a method for producing metal nanoparticles adsorbed on a carrier oxide, which method comprises the following steps

• • providing a laser ablated dispersion of metal nanoparticles in an organic solvent;
• • adding the carrier oxide to the dispersion; and
• • Adsorption of the metal nanoparticles on the carrier oxide by adding a salt that is insoluble in the organic solvent.

In the context of the present invention, nanoparticles are particles which have an average particle size of 1 to 100 nm, preferably 5 to 50 nm and particularly preferably 5 to 20 nm. Usually in the method according to the invention at least 90%, in particular 95 to 100%, of the metal particles are present as nanoparticles. The particle size of the metal particles can be determined by methods known to those skilled in the art, for example by means of transmission electron microscopy (TEM).

The metal that is supported according to the invention in the form of nanoparticles on a carrier oxide is, for example, a noble metal selected in particular from the group consisting of the platinum group metals ruthenium, osmium, rhodium, iridium, palladium and platinum, as well as gold and silver. Preferred noble metals are rhodium, palladium, platinum and gold. In addition, the metal can also be a base metal, this being selected in particular from the group consisting of rhenium, molybdenum, manganese, tungsten, niobium, tin, copper, iron, bismuth, cerium, lanthanum, barium, strontium, vanadium and antimony.

All materials familiar to the person skilled in the art for the purpose of supporting metals or precious metals can be considered as support oxides. They have a BET surface area of 30 to 250 m ² /g, preferably 100 to 200 m ² /g (determined according to DIN 66132) and are in particular aluminum oxide, silicon oxide, magnesium oxide, titanium oxide and mixtures or mixed oxides of at least two of these Materials.
Aluminum oxide, magnesium/aluminum mixed oxides and aluminum/silicon mixed oxides are preferred. If aluminum oxide is used, it is particularly preferably stabilized, for example with 1 to 6% by weight, in particular 4% by weight, of lanthanum oxide.
The carrier oxide can be added to the dispersion of metal nanoparticles in an organic solvent, for example in solid form. However, the carrier oxide is advantageously dispersed in the organic solvent in which the metal nanoparticles are also dispersed and is added in this form.

The preparation of a dispersion of metal nanoparticles in an organic solvent by means of laser ablation is a method known to those skilled in the art, see for example WO 2012/094221 A2 . Solvents which are suitable for the process according to the invention are, for example, methanol, ethanol, isopropanol, acetone, propylene carbonate, hexane, cyclohexane, octene, cyclopentanone, diethyl ether, dioxane, toluene, ethyl acetate, tetrahydrofuran, methylene glycol, ethylene glycol, triethylene glycol or a mixture of at least two of these solvents. A preferred solvent is propylene carbonate.

The concentration of the metal nanoparticles in the organic solvent is not critical per se, but a metal concentration of less than 1000 mg/l is preferred.

According to the invention, the metal nanoparticles are adsorbed on the carrier oxide by adding a salt which is insoluble in the organic solvent. Suitable salts consist of any combination of cations such as Li ⁺ , Na ⁺ , K ⁺ and NH ₄ ⁺ with anions such as SO ₄ ² ", NO ₃ , Cl ^- , Br ^- , I ^- , CO ₃ ^2- and PO ₄ ^{3 -} , salts with low lattice enthalpy being preferred Preferred salts are sodium nitrate, potassium nitrate, ammonium nitrate and potassium iodide Particularly preferred salts are potassium iodide and potassium nitrate.

The salt can be added to the dispersion of metal nanoparticles in an organic solvent, for example in solid form. However, the salt is advantageously added in the form of a stock solution in the organic solvent in which the metal nanoparticles are also dispersed. Such stock solutions can be prepared in organic solvents, for example with the aid of a crown ether. For example, stock solutions of sodium nitrate, potassium nitrate, ammonium nitrate and potassium iodide in propylene carbonate can be prepared using 18-crown-6, the molar ratio of crown ether to salt being preferably 2:1.

According to the invention, the salt is added to the dispersion of metal nanoparticles in the presence of the carrier oxide, in particular in amounts of less than 5 mmol/mg metal, preferably in amounts of 100 to 320 μmol/mg metal. According to the invention, the salt is added to the dispersion of metal nanoparticles in the presence of the carrier oxide, in particular at temperatures below the boiling point of the organic solvent used, preferably at room temperature. Likewise, the addition preferably takes place at ambient pressure.

The addition of the salt to the dispersion of metal nanoparticles in the presence of the carrier oxide is preferably carried out with stirring, with the metal nanoparticles adsorbed on the carrier oxide usually forming as a sediment in the reaction vessel within 30 to 180 minutes, which can be easily separated from the clear supernatant and dried or .can be further processed.

The metal nanoparticles produced according to the invention and adsorbed on a carrier oxide are incorporated in particular into a washcoat and coated in this form on a carrier substrate. The support substrate can be a flow-through substrate or a wall-flow filter.
A wall-flow filter is a support substrate that includes channels of length L extending in parallel between a first and a second end of the wall-flow filter which are alternately closed at either the first or the second end and which are separated by porous walls. A flow-through substrate differs from a wall-flow filter in particular in that the channels of length L are open at both ends.
In the uncoated state, wall-flow filters have, for example, porosities of 30 to 80%, in particular 50 to 75%. In the uncoated state, their average pore diameter is, for example, 5 to 30 micrometers.
As a rule, the pores of the wall flow filter are so-called open pores, which means they are connected to the channels. Furthermore, the pores are usually interconnected. On the one hand, this enables the inner pore surfaces to be easily coated and, on the other hand, an easy passage of the exhaust gas through the porous walls of the wall-flow filter.

Flow-through substrates are known to those skilled in the art, as are wall-flow filters, and are commercially available. They consist, for example, of silicon carbide, aluminum titanate or cordierite.

example 1

a) Preparation of the gold colloid

A gold sheet (thickness 1 mm) is placed in a flow chamber, through which propylene carbonate is pumped. The liquid column behind the window is 8 mm and the volume flow is 34 ml/min. A ps-pulsed laser (pulse duration 3 ps, frequency 5 MHz, power 157.5 W, pulse energy 8 mJ, wavelength 1030 nm) is focused onto the surface of the gold sheet (working distance: 21.15 cm) and scanned over it at 2 m/s. This creates a gold colloid with approx. 40 mg/l. The gold nanoparticles have an average particle size of 11.8 nm.

b) Support of the gold nanoparticles:

To support the gold nanoparticles on aluminum oxide, aluminum oxide powder is dispersed in propylene carbonate with the aid of ultrasound. The dispersion obtained is then mixed with an amount of the colloid obtained according to a) in which a mass loading of 1% by weight of gold on aluminum oxide results when the support is complete.

A quantity of potassium iodide corresponding to a concentration of approx. 280 μmol of potassium iodide per milligram of gold is then added to this mixture. The carrier efficiency reaches 80% after 30 minutes of stirring and about 90% after 180 minutes of stirring, see 1 .

example 2

Example 1 was repeated with the difference that in step b) potassium iodide was replaced by potassium nitrate in an amount of about 310 μmol milligrams of gold. 1 shows that a result comparable to Example 1 is achieved.

Example 3

Example 1 was repeated with the difference that in step b) potassium iodide was replaced by potassium nitrate in an amount of about 100 μmol milligrams of gold.

1 shows that after 180 minutes of stirring, a support efficiency very close to that of Examples 1 and 2 is achieved.

example 4

Example 1 was repeated with the difference that in step b) potassium iodide was replaced by ammonium nitrate in an amount of about 280 μmol milligrams of gold. 1 shows that after 20 minutes of stirring, a carrier efficiency of about 40% is achieved, but this has not increased even after 180 minutes.

QUOTES INCLUDED IN DESCRIPTION

This list of the documents cited by the applicant was generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Patent Literature Cited

• WO 2017/118932 A1 [0003]
• WO 2010/087869 A1 [0004]
• WO 2012/080458 A1 [0005]
• WO 2012/094221 A2 [0013]

Claims (15)

A method for preparing metal nanoparticles adsorbed on a support oxide, comprising the following steps • providing a laser ablated dispersion of metal nanoparticles in an organic solvent; • adding the carrier oxide to the dispersion; and • Adsorption of the metal nanoparticles on the carrier oxide by adding a salt that is insoluble in the organic solvent. procedure after claim 1 , characterized in that at least 90% of the metal particles are present as nanoparticles. procedure after claim 1 and/or 2, characterized in that the metal is a noble metal selected from the group consisting of the platinum group metals ruthenium, osmium, rhodium, iridium, palladium and platinum, as well as gold and silver. procedure according to claim 3 , characterized in that the noble metal is rhodium, palladium, platinum or gold. procedure after claim 1 and/or 2, characterized in that the metal is a base metal selected from the group consisting of rhenium, molybdenum, manganese, tungsten, niobium, tin, copper, iron, bismuth, cerium, lanthanum, barium, strontium, vanadium and antimony . Method according to one or more of Claims 1 until 5 , characterized in that the carrier oxide is aluminum oxide, silicon oxide, magnesium oxide, titanium oxide, or a mixture or a mixed oxide of at least two of these oxides. Method according to one or more of Claims 1 until 6 , characterized in that the carrier oxide is alumina. Method according to one or more of Claims 1 until 7 , characterized in that the carrier oxide is dispersed in the same organic solvent in which the metal nanoparticles are also dispersed and is added in this form to the dispersion of metal nanoparticles in an organic solvent of the dispersion of metal nanoparticles. Method according to one or more of Claims 1 until 8th , characterized in that the organic solvent is methanol, ethanol, isopropanol, acetone, propylene carbonate, hexane, cyclohexane, octene, cyclopentanone, diethyl ether, dioxane, toluene, ethyl acetate, tetrahydrofuran, methylene glycol, ethylene glycol, triethylene glycol or a mixture of at least two of these solvents . Method according to one or more of Claims 1 until 9 , characterized in that the organic solvent is propylene carbonate. Method according to one or more of Claims 1 until 10 , characterized in that the concentration of the metal nanoparticles in the organic solvent is less than 1000 mg/l. Method according to one or more of Claims 1 until 11 , characterized in that the salt contains any combination of cations such as Li ⁺ , Na ⁺ , K ⁺ and NH ₄ ⁺ with anions such as SO ₄ ^2- , NO ₃ ^- , Cl ^- , Br ^- , I ^- , CO ₃ ^2- and PO ₄ is ^3- . Method according to one or more of Claims 1 until 12 , characterized in that the salt is sodium nitrate, potassium nitrate, ammonium nitrate or potassium iodide. Method according to one or more of Claims 1 until 13 , characterized in that the salt is added in amounts of less than 5 mmol/mg metal. Method according to one or more of Claims 1 until 14 , characterized in that the metal nanoparticles adsorbed on a carrier oxide are incorporated into a washcoat and coated onto a carrier substrate.

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